We see that when measuring sinusoidal peaks, it is important to know
the minimum frequency separation of the peaks, and to choose an FFT
window which is long enough to resolve the peaks accurately.
Generally speaking, the window must ``see'' at least 1.5 cycles of the
minimum difference frequency. The rectangular window ``sees'' its
full length. Other windows, which are all tapered in some way
(Chapter 3), see an effective duration less than the window length in
samples. Further details regarding theoretical and empirical
estimates are given in [1].